This invention relates to a fuel cell, a manufacturing method thereof, electronic apparatus, an enzyme-immobilized electrode, a manufacturing method thereof, a water-repellent agent, and an enzyme immobilizing material, and particularly is suitable to be applied to a biofuel cell employing an enzyme and various kinds of electronic apparatus employing this biofuel cell as the power supply.
The fuel cell has a structure in which a cathode (oxidant electrode) and an anode (fuel electrode) are opposed with the intermediary of an electrolyte (proton conductor). In the conventional fuel cells, the fuel (hydrogen) supplied to the anode is oxidized to be separated into electrons and protons (H+). The electron is passed to the anode and the H+ passes through the electrolyte to move to the cathode. At the cathode, this H+ reacts with oxygen supplied from the external and the electron sent from the anode via an external circuit, to produce water (H2O).
As just described, the fuel cell is a high-efficiency electricity generating device that directly converts the chemical energy possessed by the fuel to electrical energy, and can draw the chemical energy possessed by fossil energy such as natural gas, oil, and coal as electrical energy irrespective of the use place and the use time and with high conversion efficiency. Therefore, conventionally the research and development of the fuel cell as the use application for large-scale generation of electricity and so on have been actively carried out. For example, there are track records of demonstrating that a fuel cell is mounted in a space shuttle and water for the crew can be replenished simultaneously with electrical power, and that the fuel cell is a clean electricity generating device.
Furthermore, in recent years, a fuel cell that exhibits a comparatively-low operating temperature range from the room temperature to about 90° C., such as the solid polymer fuel cell, has been developed and is attracting attention. Thus, seeking is being made for not only the use application for large-scale generation of electricity but also application to small-size systems such as the driving power supply of the car and the portable power supply of the personal computer and mobile apparatus.
As just described, the fuel cell will have a wide range of use application from large-scale generation of electricity to small-scale generation of electricity, and is attracting much attention as a high-efficiency electricity generating device. However, for the fuel cell, generally natural gas, oil, coal, or the like is converted to a hydrogen gas by a reformer and used as the fuel, and thus the limited resources are consumed. In addition, there are various problems that the fuel cell needs to be heated to a high temperature and requires a catalyst of an expensive noble metal such as platinum (Pt), and so on. Furthermore, even if a hydrogen gas or methanol is directly used as the fuel, care needs to be taken in handling it.
Thus, there has been made a proposal in which attention is paid to the fact that the biological metabolism carried out in living organisms is a high-efficiency energy conversion mechanism and it is applied to the fuel cell. Breathing, photosynthesis, and so on carried out in somatic cells of microorganisms are included in the term biological metabolism here. The biological metabolism has both of a feature that the electricity generation efficiency is extremely high and a feature that the reaction proceeds under a moderate condition at about a room temperature.
For example, the breathing is the following mechanism. Specifically, nutrients such as sugar, fat, and protein are captured into a microorganism or a cell and the chemical energy of them is converted to redox energy, i.e. electrical energy, by reducing nicotinamide adenine dinucleotide (NAD+) to reduced nicotinamide adenine dinucleotide (NADH) in the process of production of carbon dioxide (CO2) via the glycolytic system and the citric acid (TCA) cycle having a number of enzyme reaction steps. Furthermore, in the electron transport chain, the electrical energy of these NADH is directly converted to electrical energy of the proton gradient and oxygen is reduced to produce water. The electrical energy obtained therein produces adenosine triphosphate (ATP) from adenosine diphosphate (ADP) via an ATP synthase, and this ATP is utilized for the reaction necessary for the growth of the microorganism or the cell. Such energy conversion is carried out in the cytosol and the mitochondrion.
Furthermore, the photosynthesis is a mechanism in which water is oxidized to produce oxygen in the process of capturing light energy and converting it to electrical energy by reducing nicotinamide adenine dinucleotide phosphate (NADP+) to reduced nicotinamide adenine dinucleotide phosphate (NADPH) via the electron transport chain. This electrical energy captures CO2 and is utilized for carbon fixation, and is utilized for carbohydrate synthesis.
As a technique for utilizing the above-described biological metabolism for the fuel cell, a microbial cell has been reported in which electrical energy generated in a microorganism is drawn outside the microorganism via an electron mediator and this electron is passed to the electrode to thereby obtain a current (refer to e.g. Japanese Patent Laid-open No. 2000-133297).
However, in microorganisms and cells, not only the intended reaction such as conversion from chemical energy to electrical energy but also many unnecessary reactions exist. Therefore, in the above-described method, electrical energy is consumed by undesired reactions and thus sufficient energy conversion efficiency is not exerted.
Thus, a fuel cell (biofuel cell) that carries out only the desired reaction by using an enzyme has been proposed (refer to e.g. Japanese Patent Laid-open No. 2003-282124, Japanese Patent Laid-open No. 2004-71559, Japanese Patent Laid-open No. 2005-13210, Japanese Patent Laid-open No. 2005-310613, Japanese Patent Laid-open No. 2006-24555, Japanese Patent Laid-open No. 2006-49215, Japanese Patent Laid-open No. 2006-93090, Japanese Patent Laid-open No. 2006-127957, Japanese Patent Laid-open No. 2006-156354, Japanese Patent Laid-open No. 2007-12281, Japanese Patent Laid-open No. 2007-35437, and Japanese Patent Laid-open No. 2007-87627). This biofuel cell breaks down the fuel by the enzyme to separate the fuel into protons and electrons, and one that employs an alcohol such as methanol or ethanol or a monosaccharide such as glucose as the fuel has been developed.